Battery charging commonly occurs in millions of remote cabinets, office buildings and electric vehicles. Unfortunately, battery charging has resulted in numerous explosions that destroy the battery and its container and possibly injure or kill people. These explosions are often caused by igniting hydrogen in insufficiently ventilated battery chambers.
When batteries are overcharged, the current in the cell produces hydrogen. Hydrogen becomes an explosion hazard when its concentration rises to about 4% volume in air, generally referred to as the lower explosive limit (LEL). To avoid this hazard, battery chambers are typically vented to release the hydrogen to the environment. Ventilation can be generated by active venting systems such as small fans located within the battery chamber or large passive vents that allow flow by convection and diffusion between the inside gas and the outside air.
Passive ventilation systems generally work by taking advantage of the fact that ambient air is denser than hydrogen. Thus, hydrogen released into a battery chamber by overcharging batteries lessens the density of the gas causing it to rise within the chamber. Typically, the chamber will have one lower and one upper vent so that the rising gas within the chamber flows out through the upper vent. Fresh outside air will then flow through the lower vent to replace the lost gas in the chamber.
One problem with existing passive ventilation systems is that they are typically sensitive to temperature differences between the air inside the battery chamber and the outside environment. Warmer air tends to rise in the same manner as lower density air. If the inside temperature is cooler, for example, the cooler air will be urged downward thereby counteracting the convective flow caused by the lower density hydrogen in the chamber. In fact, if the inside temperature is substantially cooler (on the order of 2%), convective flow completely stops and the hydrogen concentration will slowly build-up in the chamber. If the inside temperature is warmer, the inside gas will rise causing unwanted ventilation when there is no hydrogen present in the chamber.